1. Field of the Invention
The present invention generally relates to digital oscilloscopes, and more particularly to a digital oscilloscope having an improved peak detect mode of operation.
2. Discussion of the Related Art
Oscilloscopes have been used for years for a number of purposes, including monitoring waveforms of various data, among other purposes. They have become an essential for research, development, and manufacturing of electronic devices. Analog oscilloscopes provide a continuous time-based display of the instantaneous amplitude values of electrical phenomena, and are thus able to accurately display the waveforms of complex signals, such as high-frequency carrier signals having low-frequency envelopes. Analog oscilloscopes, however have a significant disadvantage in that they are unable to store signal waveforms. Also, when viewing very fast signal components, it is difficult to view spikes or glitches due to the dim scope illumination.
With the progress of digital technology, digital oscilloscopes have been developed allowing them to store signal waveforms. Digital oscilloscopes chop input signals into discrete time points determined by an internal clock, quantize the instantaneous amplitude values at those points, and store the resulting digital representations in a digital memory. The display is regenerated from memory at a predetermined clock rate, and is manifested either as a series of dots, or connected dots (i.e., vector mode). Since the input signals are not functionally related to the internal clock of the digital oscilloscope, whatever the instantaneous value of the input signal happens to be when the clock edge occurs is what gets stored.
To address this shortcoming, digital oscilloscopes typically include a xe2x80x9cpeak detectxe2x80x9d mode of operation. In this mode, an analog input signal is sampled at high speed, and converted into digital data by an A/D converter. Maximum peak amplitudes (or maximum and minimum values) of the digital data in a predetermined time period are sequentially stored in memory for later retrieval and display. In this manner, high frequency noise or narrow signal glitches, which are virtually impossible to be detected by general sampling technology, can be detected and stored for later retrieval and display. With a peak detect mode of operation, signal envelopes can be measured, and aliasing can be detected. The detection and storage of peak values (both maximum and minimum) is known and understood by those skilled in the art.
However, one problem that results from peak detection is that it exaggerates noise in the signal, including noise internal to the digital oscilloscope, and displays worst case noise performance. This is because any noise peaks occurring anywhere on the waveform are likely to be acquired and the worst case noise performance is displayed. While sometimes this worst case noise is the information desired, it often masks desired statistical information. For this reason, Peak Detect is a special mode on many digital oscilloscopes, and is often enabled (or selected) only when needed. Another shortcoming of Peak Detect mode is that it typically loses statistical information in the waveform.
As is known and taught in U.S. Pat. No. 5,740,064, assigned to the assignee of the present invention, the exaggerated noise shortcoming in the Peak Detect mode may be addressed by a mechanism that selectively displays either the actual sampled and decimated data or the peak data, during a decimation period. By way of strict, traditional definition, a decimated data point every nth data point, where there are n data points in a decimation period (usually one display pixel column). A xe2x80x9cditheredxe2x80x9d data point refers to one of every n data points. A dithered data point may be every nth sample point, or it may simply be one (statistically) of every n sampled points. Unless expressly stated as every nth data point, the term decimated data point (or value), as used herein, will refer to one of every n data points. In this regard, the term decimated data point, as used herein, will generally be synonymous with dithered data point.
In keeping with the discussion, the actual sampled (and decimated) data is usually displayed. However, when the difference between the maximum and minimum peak values in a given decimation period exceeds a predetermined threshold (indicating a significant short-term change), then the peak data is displayed. In this way, baseline noise is not exaggerated, but significant signal glitches are captured and displayed.
More specifically, the peak detect technique of U.S. Pat. No. 5,740,064 includes the step of processing a series of digital signal samples through a decimator to extract a decimated sample value from each decimation period. The series of digital signal samples is simultaneously processed through a digital peak detector to extract maximum and minimum values (peak detect sample values) from each decimation period. For a given decimation period, a difference between the maximum and minimum sample values for the interval is calculated. If the difference exceeds a glitch detect threshold value, the maximum and minimum sample values for the given decimation period are transferred to a video sample memory. If not, the decimated sample value for the given decimation period is transferred to the video sample memory.
While the system disclosed in U.S. Pat. No. 5,740,064 effectively de-emphasizes baseline noise, further improvement is desired. In this regard, the system of U.S. Pat. No. 5,740,064 displays either the sampled, decimated values or peak values, for a given decimation period. Consequently, one of these values is neglected (i.e., not displayed) for each decimation period. Thus, some information, which may be valuable to a user, is being withheld. In addition, the threshold of this method adds distortion when plotting signal components of magnitude close to the threshold.
Accordingly, it is desired to provide an improved digital oscilloscope having a peak detect mode that overcomes the shortcomings mentioned above.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the advantages and novel features, the present invention is generally directed to a digital oscilloscope that provides a peak detect mode of operation that not only accurately displays peak information, but also reflects the significance of peak information. In this regard, and in accordance with one aspect of the invention, the oscilloscope includes an analog to digital (A/D) converter configured to periodically sample an input signal and convert the periodic samples into a train of digital data. A circuit is disposed at the output of the A/D converter and is configured to evaluate the train of digital data over a first predetermined period of time and detect a minimum value and a maximum value during the first period of time. A mechanism is configured to evaluate the minimum and maximum values and determine a significance measure of the minimum value and the maximum values. Finally, the oscilloscope includes a display controller configured to vary an intensity of a signal representation on a display, in response to the significance measure. Additionally, it is assumed that a mechanism is provided that can also plot a statistically valid view of the sampled waveform, resulting in a composite view that includes both peak and statistical information. In this regard, this composite view may be formed by adding the individual intensities of the peak and statistical view intensities. Alternatively, it may simply pick the brightest intensity, as between the peak view intensity and the statistical view intensity, and plot the composite view at that brightest intensity.
In operation, the oscilloscope samples an electric signal at a periodic sample rate. For a given decimation period, which typically comprises a plurality of sample points, the scope saves the maximum and minimum values (i.e., peak information) of the signal. Assuming that the oscilloscope is operating in vector mode, wherein consecutive points are connected, the oscilloscope displays not only the decimated data points in each decimation period, but also the peak data as well. An important aspect of the present invention, however, is that the peak data is not displayed in such a fashion as to emphasize noise. In this respect, peak data is displayed with a varying display intensity, such that the lower the significance measure of a peak value, the lesser the intensity. The significance measure of a peak value (i.e., max/min values of a decimation period) may be determined in a number of different ways, and may be based on a number of different factors. For example, the peak values may be compared to the other sampled values in that decimation period. If the peak values differ by a large amount, then they may be deemed as either more significant, or less significant, depending upon the mode of operation. Similarly comparisons may be made by comparing the peak values against peak values in neighboring decimation periods.
As will be better appreciated from the detailed description that follows, the present invention enjoys several advantages over the systems known in the prior art. For example, the oscilloscope of the present invention preserves the peak information for all decimation periods, yet displays that information in such a way (e.g., variable intensity) that noise is not amplified and statistical information is displayed and not obscured by peak information, where the peak information does not add significant new information. The invention may be configured to emphasize narrow events and level transitions, which characteristics have traditionally been difficult to see on analog scopes, or even digital scopes in dots rendering mode.
In accordance with another aspect of the invention, a method is provided for implementing a peak detect mode of operation on a digital oscilloscope. In accordance with this aspect of the invention, the method includes the step of converting an input signal into a train of digital data. The method then evaluates the train of digital data over a first predetermined period of time and detecting a minimum value and a maximum value during the first period of time. Then, the method determines a significance measure of the minimum value and the maximum value. Finally, the method displaying the peak value (or min/max) on a display of the oscilloscope. Preferably, this displaying step includes varying the display intensity based upon the significance measure of the minimum value and the maximum value.